1. Field of the Invention
The present invention relates to a method of forming a polycrystalline silicon thin film with improved electrical characteristics and a method of manufacturing a thin film transistor using the method.
2. Description of the Related Art
A general liquid crystal display uses an amorphous silicon thin film transistor (a-Si TFT) as a switching element. In recent years, with the demand for high-definition display quality LCDs, a polycrystalline silicon thin film transistor (poly-Si TFT) having a high driving speed is widely used.
In the poly-Si TFT, various methods for forming the poly-Si thin film are well known. Of the different types of methods for forming poly-Si, the poly-Si thin film may be formed directly on a substrate made of, for example, glass. Second, the poly-Si thin film may be formed by forming an amorphous-Si (a-Si) thin film and then annealing the same.
In general, a glass substrate used in an LCD may deform during a general annealing treatment process performed at about 600° C. Thus, an excimer laser annealing (ELA) process has been suggested in order to anneal the a-Si thin film. According to the ELA process, laser beams having high energy are irradiated on the a-Si thin film. That is, the a-Si thin film is instantaneously melted for several nanoseconds (ns) and recrystallized without causing damages to the glass substrate.
In addition, the ELA process provides a Si thin film having a relatively high electric mobility such that Si atoms are rearranged in grains having high crystallinity when the a-Si thin film is melted in a liquid phase and then solidified.
In amorphous silicon thin film transistors (a-Si TFTs) used as switching devices of conventional liquid crystal displays (LCDs), an amorphous silicon thin film is crystallized into a polycrystalline silicon thin film by conventional excimer laser annealing. However, the polycrystalline silicon thin film is composed of grains with all crystalline plane orientations, i.e., with no regularity of crystalline plane orientations.
Generally, it is known that the {110} or {111} crystalline plane of polycrystalline silicon exhibits electrical mobility of about 300-400 cm2/V·s, whereas the {100} crystalline plane of polycrystalline silicon exhibits electrical mobility of about 600 cm2/V·s. For example, when a polycrystalline silicon thin film has grains predominantly oriented in the {100} crystalline plane direction, electrical mobility can be enhanced by about 1.5-2 times.
Thus, in order to enhance electrical characteristics of a polycrystalline silicon thin film transistor, it is necessary to form grains with selectively specific crystalline plane orientation.